Recently, in various types of automotive vehicles such as ordinary automobiles, trucks and the like, component parts such as chassis members, vehicle main-frame members, bumpers and the like, which are made of aluminum or aluminum alloys, have been widely used in place of those which are conventionally made of steel, because the aluminum chassis members, etc., are superior, especially with respect to the light-weight of the vehicle main-frame members, prolonged service life of vehicles, recyclability, etc.
In manufacturing such kinds of vehicle component members, it is an ordinary practice to employ the extrusion technique. The reason for this is that the melting point of aluminum, which serves as a raw material, is low. In such an extrusion technique, an extrusion die set having a hole portion which has a configuration similar in section to those of the vehicle component members is firmly secured to a distal end portion of a container, a billet is inserted into the interior of the container, and then the billet is pressed towards the extrusion die set by a stem so that the billet is extruded out of the hole, thereby appropriately shaping the above-mentioned vehicle component members. According to this extrusion technique, since the hole portion of the extrusion die set has a constant sectional configuration, the vehicle component members thus obtained each has a constant sectional configuration in the longitudinal direction.
Incidentally, these various types of component members use H-shaped and T-shaped members consisting essentially of a web and a flange portion because these shapes offer predetermined amounts of mechanical strength, in other words, high sectional secondary moment. However, the employment of the H-shaped and the T-shaped members necessitates an increased outer configurational dimension in section and the requirement for smaller installation space cannot be met. Moreover, the flange portion sometimes interferes with other attachment members depending on where such H-shaped and T-shaped members are attached. Therefore, research is being conducted regarding the use of a tubular member which is smaller in outer configurational dimension and has superior mechanical strength.
However, in the conventional extrusion die set of the type mentioned above, since its hole portion has a constant sectional configuration, a resultant-molded tubular member also has a constant sectional configuration in the longitudinal direction. In a component member of this type, however, distribution of bending stress to be acted thereon usually varies in the longitudinal direction and the component member is obliged to have larger dimensions and strength than necessary at portions on which no large bending stresses act. Accordingly, molding material is excessively and wastefully used. This is particularly inefficient economically. Moreover, the requirements for the originally intended smaller installation space and light-weight design cannot be met.
As one way to hopefully avoid the above problems, employing an extrusion molding die set of the type as disclosed, for example, in International Publication No. WO93/00183, may be contemplated.
FIGS. 10 and 11 depict a conventional extrusion molding die set disclosed in the above International Publication. This extrusion molding die set is originally designed to extrude a tube having a variable shape and a variable wall thickness and which tube is made of copper. In the illustration, reference numeral 1 denotes a container; 2, a mandrel; 3, a pressure ram; and 4, a billet, respectively. The container 1 is provided at a distal opening portion thereof with a die set 5. The die set 5 is provided with a die section variable device 7. This die section variable device 7 has a hydraulic cylinder 8 for causing a plate-like extrusion member 7 to move in and out of the die set 5.
According to the above extrusion molding die, the hydraulic cylinder 8 of the die section variable device 6 is properly operated to cause the plate-like extrusion member 7 to move in and out of the die set 5 while extruding the billet 4 in the container 1 from the die set 5 with the pressure ram 3, so that a variable section tubular member 10, which has projections 9 on desired areas of an outer peripheral portion, can be formed, as shown in FIG. 11.
However, any attempt to apply of the above conventional extrusion molding die set to the technique for molding various kinds of tubular component members of the types mentioned above encounters problems in that since the projections 9 are formed merely on the area where mechanical strength is required in spite of the inside diameter (inner configurational dimension) being kept constant by the mandrel 2 in the longitudinal direction, the wall thickness of this area becomes larger than necessary due to the provision of the projections 9, and as a result, molding material is wastefully consumed and the requirement for making the component members light in weight cannot be met. Moreover, it gives rise to another problem in that the projections 9 locally formed are obstructive in view of the attachment structure.
The present invention has been accomplished for the purposes of effectively obviating the problems inherent in the conventional extrusion die set and the extrusion molding method using this die set. It is, therefore, an object of the present invention to provide a variable section extrusion die set and a variable section extrusion molding method capable of molding a tubular member which is arbitrarily varied in outer configurational dimension and inner configurational dimension in the longitudinal direction when a molding material such as aluminum is to be extruded.